This invention relates to optical time domain reflectometry and more specifically to a subset of a micro hand-held instrument used for examination of singlemode optical fibers in passive optical networks (PON).
Optical time domain reflectometers (OTDR) are used in the telecommunications industry for testing, troubleshooting, and characterizing fiber optic cables to measure fiber cabling properties such as breaks, connector loss, splice loss, fiber attenuation, attenuation coefficients, fiber length, and other parameters that affect the signal transmission quality through the cable.
In examining a fiber with an OTDR, optical pulses are launched into the fiber under test from a pulsed laser diode at a low duty cycle. During the time between the optical pulses transmitted, returned light from the fiber under test is reflected back in the form of backscatter and reflections associated with events, such as connectors, and is then converted to an electrical signal, and processed for display. The signal is displayed as an amplitude in decibels versus a length plot showing a gradually decreasing backscatter energy level with reflective events appearing as pulses on the backscatter.
There are two major types of instruments using OTDR technology. The first is the fully functional mini-OTDR and the second is the reduced functionality optical fault finder. Both use backscatter, reflection of signals or waves back to their source, to make measurements. A third type of instrument using OTDR technology is the reflection finder that does not use backscattering. This type of instrument is very low cost, has relatively high spatial resolution but can not locate non reflective events such as an open APC (angle polish connector).
The mini-OTDR is a more complicated, higher cost instrument used by trained technicians to troubleshoot fiber cables. During the PON installation, mini-OTDRs are deployed to verify that proper installation was done during the construction phase and that quality of service is high. These OTDRs have high dynamic range needed to overcome the loss of the optical splitters used to distribute feeder fibers to individual customers. These OTDRs also have high spatial resolution so that the reflectance emanating from each customer's optical network terminal can be separated and measured. The design tradeoffs of an optical fault finder, such as trading off cost reduction for spatial resolution, make this type of instrument not a good alternative for PON testing. The third type of tester not using backscatter has the disadvantage of only finding reflections along a fiber. While this is useful for locating a fault, it does not provide enough information to make this type of tester viable for PON testing. In the PON network it is required to make measurements with backscatter such as locating an open APC. An APC provides little if any reflectance. Normally, an open connector having a different type of polish such as ultra physical contact (UPC) will provide a very strong reflection. In PON, an open APC at a distribution point must be identified.
There is a need for an easy to use, low cost, high resolution, palm-size instrument for testing singlemode PON networks between the customer and the first distribution point that provides connectivity information or the location of an open. This instrument could be used after the construction phase, during maintenance, or for simple troubleshooting. It could be more widely deployed, requiring less skill to operate. The skilled OTDR technician would only be needed to handle the more complicated problems.